[Bug c++/106734] New: [requires] std::same_as in compound requirements doesn't produce expected result

[Bug c++/106734] New: [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

            Bug ID: 106734
           Summary: [requires] std::same_as in compound requirements
                    doesn't produce expected result
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakob at schmutz dot co.uk
  Target Milestone: ---

On gcc version 12.2.0 (and 11.3.0) I would expect the following code


```
#include <iostream>

struct Bar {
};

int main()
{
    Bar bar;
    constexpr bool same = requires
    {
        { bar } -> std::same_as<Bar>;
    };

    constexpr bool conv = requires
    {
        { bar } -> std::convertible_to<Bar>;
    };

    if constexpr (std::same_as<decltype(bar), Bar>)
    {
        std::cout << "AS SAME" << std::endl;
    }

    if constexpr (std::is_same_v<decltype(bar), Bar>)
    {
        std::cout << "IS SAME" << std::endl;
    }

    if constexpr (same)
    {
        std::cout << "REQUIRES SAME" << std::endl;
    }

    if constexpr (conv)
    {
        std::cout << "CONVERTIBLE" << std::endl;
    }

    return 0;
}
```

to return

```
AS SAME
IS SAME
REQUIRES SAME
CONVERTIBLE
```

based on how how [compound
requires](https://en.cppreference.com/w/cpp/language/requires) section reads


but instead it returns

```
IS SAME
REQUIRES SAME
CONVERTIBLE
```

compiled with `g++ temp.cpp -std=c++20`

compiling with `clang++ temp.cpp -std=c++20` produces the expected output

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #1 from jakob at schmutz dot co.uk ---

> but instead it returns
> 
> ```
> IS SAME
> REQUIRES SAME
> CONVERTIBLE
> ```


whoops this was supposed to be

```
AS SAME
IS SAME
CONVERTIBLE
```

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #2 from jakob at schmutz dot co.uk ---
Created attachment 53503
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53503&action=edit
preprocessed file

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to jakob from comment #0)
>     Bar bar;
>     constexpr bool same = requires
>     {
>         { bar } -> std::same_as<Bar>;

This is false. The type of decltype((bar)) is Bar&.

So I think GCC is correct.

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #4 from jakob at schmutz dot co.uk ---
(In reply to Jonathan Wakely from comment #3)
> (In reply to jakob from comment #0)
> >     Bar bar;
> >     constexpr bool same = requires
> >     {
> >         { bar } -> std::same_as<Bar>;
> 
> This is false. The type of decltype((bar)) is Bar&.
> 
> So I think GCC is correct.



Right but then why is `std::same_as<decltype(bar), Bar>` true? Shouldn't that
also be false then?

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #5 from jakob at schmutz dot co.uk ---
oh I see `std::same_as<decltype((bar)), Bar>` is false

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[jakob at schmutz dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

jakob at schmutz dot co.uk changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #6 from jakob at schmutz dot co.uk ---
Will close then as I agree that GCC is correct

(I just found it a little confusing)

[Bug c++/106734] [requires] std::same_as in compound requirements doesn't produce expected result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106734

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
decltype(bar) and decltype((bar)) are not the same type.